1. Field of the Invention
This invention relates to electronic text display and processing systems which in their normal mode display only a partial page of text. More particularly this invention is a device for reducing in size a full page of text so as to fit onto the visual display screen of a partial-page display word processing device without impairing the word processing functions of the device.
2. Description of the Prior Art
Prior art electronic word processing and text display systems utilizing a visual display screen, such as those exemplified by U.S. Pat. No. 3,810,107, generally have rather large displays of the cathode ray tube type and are capable of displaying a full page (81/2 inches by 11 inches) of text at one time. The display of an entire page of text allows an operator to visually check and correct the format and layout of the page as a whole, thus simplifying the task of preparing aesthetically pleasing documents.
Smaller less expensive word processing systems have been developed since the introduction of the original large display screen devices. The component most reduced in size in the smaller word processing systems is the display screen, which is commonly reduced to 10 inches wide by 5 inches high. These systems display only a portion of a page of text at one time. A major problem of a small display screen system is that checking and correcting the format of the full page of text is difficult when only a portion of the page, such as one-third, is viewable at any one time.
Several potential solutions exist to the problem of reducing a page of text to fit within the confines of the display screen of a partial-page word processing system. The major difficulty encountered in such a reduction is in shrinking the horizontal dimension of the page so that the aspect ratio of the page remains constant. One solution would be to increase the output rate of the characters in the text, resulting in the same amount of information being displayed in a shorter length. However, this solution would require faster and more costly electronic logic circuits and a digital video system having a wider than normal information bandwidth.
Other possible horizontal reductions involve the horizontal sweep circuit of the cathode ray tube display assembly. As a general rule reducing the current through the horizontal winding of the deflection yoke compresses the horizontal image produced on the cathode ray tube screen. However, unless the method used to limit the current is carefully designed, adverse side-effects may occur affecting other functions of the horizontal sweep circuit, such as the beam retrace period. Another adverse side effect would be serious overstressing of other circuit components.
One prior art device employing switchable changes in the level of current to the deflection circuits of the cathode ray tube is a consumer-oriented television set with a "zoom" feature that enlarges the picture. Increases in the current to both the horizontal and vertical deflection circuits are made, thus expanding the transmitted picture. However, this technique is not practical for image size reduction in high resolution character displays as used in word processing machines.
Prior art systems that allow for reductions in image sizes, such as those exemplified by U.S. Pat. No. 3,976,982, by means of logical area-averaging techniques, are unsuitable for horizontal compression in a word processing device due to a loss in resolution of the characters comprising the text.
An object of the present invention is therefore to provide a means for achieving both vertical and horizontal compression of digital display data which does not require costly exotic circuitry, cause adverse side effects on other circuit components, and which is easily implemented on an existing display machine and readily selectable by the machine operator.
Other objects and advantages of the present invention will become apparent as the device is described below.